The Compton Effect Introduction

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PHY 192 
Compton Effect Spring 2012 
1 
The Compton Effect 
Introduction 
Light is made up of (many) photons. “Photon” is the name given to the particle-like aspect of light’s 
behavior (in addition to the wave-like behavior you have previously measured). In this experiment we will 
study two specific aspects of the interaction of photons with electrons. The first of these is the Compton effect, 
named after Arthur Holly Compton who received the Nobel Prize for physics in 1927 for its discovery. The 
other effect deals with the radiation emitted when a tightly bound electron from a heavy element is kicked out 
by a photon. This gives rise to “characteristic” X-rays that can be used to identify the element. 
This experiment uses material from the Introduction to Nuclear Radiation. 
Theory of the Compton Effect 
Kinematics of the Compton Effect 
The Compton effect is based on treating light as consisting of particles of a given energy related to the 
frequency of the light wave. In this context, the particle of light is given the name “photon”. An energetic 
photon with energy of 0.1 MeV (million electron volts) or larger is also often referred to as a gamma ray. An 
MeV is an energy unit, equal to the kinetic energy an electron would gain by being accelerated through a 
voltage difference of 1 MV (10
6
volts). Photons whose energy is in the range of 0.1 to 100 keV are usually 
referred to as X-rays (1 keV = 10
-3
of 1 MeV). 
If a photon with energy Eo strikes a stationary electron, as in Figure 1, then the energy of the scattered photon, 
E, depends on the scattering angle, 
Θ
, that it makes with the direction of the incident photon according to the 
following equation: 
Cos Θ
1
(1) 
where m
e
is the mass of the electron and m
e
c
2 
= 511 keV = .511 MeV. In real matter, electrons are not 
stationary, but if the initial kinetic energy of the electron is small compared to the energy of the incoming 
photon, Eq. 1 will describe the situation well. This will be the case for gamma rays (E > 10
5
eV) scattering off 
outer electrons of atoms (typical kinetic energy of a few eV).
Fig. 
1: 
Schematic 
diagram 
of Compton Effect kinematics. 
 
E0
E
Θ
Ε
e

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PHY 192 
Compton Effect Spring 2012 
2 
The derivation of this equation is based on applying special relativity and kinematics to the photon as a 
quantum of light, but in the form we use, requires little or no reference to the wave nature of light! It’s all 
about energy of the electron and photon, and the angle of the outgoing photon compared to the initial direction. 
The total energy of the electron E
e
is the sum of its kinetic energy T
e
and its rest energy m
e
c
2
, 
i.e. 
E
e 
= 
T
e 
+ m
e
c
2
. The total